This invention relates to a process for forming a protective oxide film in-situ after deposition of an epitaxial silicon layer on a silicon substrate wafer.
Epitaxial deposition is a film grown over a crystalline substrate in such a way that the atomic arrangement of the film bears a defined crystallographic relationship to the atomic arrangement of the substrate wafer. In the case of a monocrystalline substrate wafer, the crystallographic orientation of the epitaxial layer will replicate that of the substrate wafer wherein the substrate wafer provides the crystallographic seed for epitaxial growth.
Commonly, growth of an epitaxial layer is accomplished by chemical vapor deposition (CVD) at temperatures well below the melting point of either the substrate wafer or the film being deposited. In the CVD technique, the substrate wafer is heated in a chamber into which reactive and carrier gases are introduced. For silicon deposition, reactive gases include Silane (SiH4), Dichlorosilane (SiH2Cl2), Trichlorosilane (SiHCl3), and Silicon Tetrachloride (SiCl4), with dopant gases that include Arsine (AsH3), Phosphine (PH3), and Diborane (B2H6), and a carrier gas of hydrogen.
Epitaxial reactors are generally available in three basic designs. The first design involves placing the substrate wafers on holders, called susceptors, in a horizontal position. Reactive and carrier gases are then introduced into the growth chamber at one side, passed over the substrate wafers, and exhausted out the other side. The second design employs a vertical system wherein the substrate wafers are placed horizontally on a rotating susceptor, and the gases are introduced into the chamber at the top, passed over the wafers, and exhausted out of the chamber at the bottom. Finally, the third design places the wafers near vertically on a barrel-type rotatable susceptor, with the gases introduced in the top of the chamber, passed over the wafers, and exhausted out the bottom of the chamber. Older technology produced multiple wafers simultaneously, and utilized each of these three designs. Newer technology, however, typically processes wafers individually, and employs the first general design wherein the wafer is placed horizontally on a rotating susceptor, and the gases are introduced at one side of the chamber, passed over the wafer, and exhausted out the other side.
In each design, the susceptor is made of a nonreactive material capable of enduring extreme temperature and pressure variations, such as graphite, and typically silicon carbide coated graphite. Heat is typically supplied by radio frequency (RF), ultraviolet (UV), infrared radiation (IR), or electrical resistance heaters, with processing temperatures ranging from about 900xc2x0 C. to 1200xc2x0 C.
In general, epitaxial deposition begins by loading the substrate wafer(s) onto the susceptor, and purging the ambient air out of the reaction chamber by supplying non-reactive gases such as helium, argon, or nitrogen, into the chamber. The temperature is then ramped up to the desired level, and a mixture of the carrier gas and the reactive gases (including any desired dopant gas) is introduced into the chamber. When the desired epitaxial layer thickness is achieved, non-reactive gases are reintroduced into the chamber, and the temperature is ramped down. The wafer is then unloaded from the chamber.
If desired, an etching agent such as anhydrous hydrogen chloride (HCl) can be introduced before carrier and reactive gases are introduced. This etching agent will remove a thin layer off the surface of the substrate wafer, as well as any contaminants adhered thereto. After such an etch, a contaminant free substrate surface with strong crystallographic structure is provided for epitaxial deposition, and generally results in a higher quality epitaxial layer. This etching step can also be employed without the substrate wafer present, as a means of controlling epitaxial deposition on the susceptor or other surfaces in the growth chamber. Additionally, prior to epitaxial deposition, a hydrogen bake can be used to remove any native oxide growth on the surface of the wafer, by chemical reduction. This will provide a clean silicon surface on the substrate for epitaxial deposition.
In the case of a silicon epitaxial layer deposited on a silicon substrate wafer, the surface of the epitaxial layer is hydrophobic. Such a hydrophobic layer is very reactive, and prone to attract contaminants. As such, it is common in the industry to employ a wafer cleaning and oxidizing step after the epitaxial deposition is complete. This cleaning and oxidizing is done to remove any contaminants that might have adhered to the epitaxial surface upon being removed from the deposition chamber, and to put a protective oxide layer, such as silicon dioxide (SiO2) on the surface of the epitaxial layer. An oxide layer surface is hydrophilic, which is much less reactive than a hydrophobic surface, and therefore does not as readily attract contaminants. The oxide layer is therefore used to protect the surface of the wafer from contaminants until the wafer is ready for further processing, wherein the oxide layer is removed and the silicon epitaxial layer is exposed and ready for processing.
This cleaning and oxidizing step adds both processing time and cost to the production of the wafer and requires additional equipment and chemical usage. It is well known in the industry to use a wet chemical bench to clean and oxidize the wafer surface. A typical cleaning and oxidizing process involving subjecting the wafer to submersion in two sequential solutions is as follows:
NH4OH (29 weight %)+H2O2(30%)+DI H2O at 70-80xc2x0 C.; and
HCl (37 weight %)+H2O2(30%)+DI H2O at 75-80xc2x0 C.
Subjecting wafers so these solutions will slightly etch the surface of the wafer to remove contaminants, and then generate a thin oxide layer. This method of oxidizing is relatively uncontrolled however, and the thickness of the oxide layer is hard to control and predict.
The present invention relates to a process that overcomes the disadvantages and problems set forth above. More specifically, a process is provided for growing an outer protective layer on the outer surface of a semiconductor wafer directly in an epitaxial reactor chamber immediately after epitaxial deposition. The subject process involves the growing of protective films in reactors designed explicitly for the deposition of epitaxial silicon films. The growth of these protective films is accomplished during typically an unproductive part of the deposition cycle, namely, the cool-down phase. In any case, the oxidation occurs before the wafer is removed from the epitaxial deposition equipment.
By incorporating the novel process technique of the present invention into the epitaxial reaction sequence, the elimination of the costly and time-consuming cleaning and oxidizing step will result. Further, since the oxidation occurs in the epitaxial equipment, the process can be much more tightly controlled, and will result in a higher quality oxide.
Additionally, by incorporating the process of the present invention, the application of an outer layer on the epitaxial silicon is not limited to an oxide, but could also include nitrides, or other beneficial layers. Nitride layers cannot currently be achieved through any presently available wet treatment technique.
These features are believed to be a novel approach utilizing existing epitaxial deposition equipment, and applying a new method. The invention discloses a method that will allow for the elimination of post-epitaxial wet processing for cleaning and oxidizing in preparation for wafer storage. More specifically, the method comprises introducing a monocrystalline substrate wafer into epitaxial equipment, processing the wafer to form an epitaxial layer on the surface of the substrate wafer and having the same crystalline properties as the substrate wafer, and then forming a protective layer on the surface of the epitaxial layer before removing the wafer from the epitaxial equipment.
The semiconductor wafer substrate is introduced into the epitaxial layer process chamber, and the temperature in the epitaxial layer process chamber is increased to a predetermined operating temperature. Typical operating temperature during epitaxial deposition ranges from about 1025xc2x0 C. up to about 1150xc2x0 C., and preferably from about 1050xc2x0 C. to 1100xc2x0 C. Typical operating pressure is about 760 Torr for atmospheric deposition, but can go down to about 1 Torr for low pressure applications. Upon achieving the desired temperature, appropriate reactive and carrier gases are introduced into the epitaxial deposition chamber to facilitate layer growth. The operating temperature is maintained for a time sufficient to facilitate epitaxial layer growth of a layer up to 15 microns thick, preferably in the range of 2 to 4 microns thick. Upon achieving the desired layer thickness, the reactive gas supply is terminated, and the deposition chamber is purged with non-reactive gas. As previously described, the epitaxial layer is hydrophobic in nature, is of uniform thickness, and has the same crystallographic orientation as that of the substrate wafer.
The temperature of the deposition chamber is then ramped down to a desired unload temperature. While ramping down the temperature, a chemical reagent gas is introduced into the epitaxial deposition chamber. Preferably, the step of introducing the reagent gas into the deposition chamber occurs immediately after purge gas is introduced into the chamber. In another preferred embodiment, the operating temperature is ramped down to a predetermined reduced temperature before introduction of the reagent gas.
When the reagent gas is introduced into the deposition chamber, the reagent gas reacts in-situ with the hydrophobic epitaxial layer surface to form an outer layer that is substantially hydrophilic. In one preferred step the substantially hydrophilic outer layer surface is an in-situ oxidation step. In another preferred step the substantially hydrophilic outer layer surface is an in-situ nitridation step. The reaction between the reagent gas and the hydrophobic epitaxial layer is preferably conducted without any additional heat being added to the deposition chamber, but rather takes advantage of the heat present from the epitaxial deposition during the temperature reduction step. Yet another additional process step preferably includes the step of subsequently depositing in-situ at least one additional layer onto the substantially hydrophilic outer layer.
In any case, it is preferred that the step of reacting the reagent gas with the hydrophobic epitaxial layer to form the hydrophilic layer is conducted without requiring substantial additional process time compared to the epitaxial deposition process described above.
In an alternate embodiment, a stabilized heat controlled process could be added to the epitaxial deposition process specifically added to support the formation of the protective hydrophilic layer. In this case, the process time required for processing in the epitaxial equipment is slightly increased, such as for an additional 20 seconds of controlled heating. By controlling the temperature during formation of the hydrophilic layer, more precise control of the layer thickness and uniformity can be achieved. Again, the formation of the hydrophilic layer is accomplished in-situ after deposition of the epitaxial layer, and before the wafer is removed from the epitaxial equipment.
The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment that proceeds with reference to the drawings.